When galvanized steel sheets are laser welded at an overlap joint, a defined gap is typically provided between the metal sheets so that resultant zinc vapors, which may disrupt the operation, can dissipate. In practice, that gap dimension between the metal sheets may not always be complied with in a reproducible manner. If the permissible gap dimension is exceeded by only a few tenths of a millimeter, a lack of fusion may be produced between the metal sheets. This lack of fusion is often referred to as a “false friend” because the weld seam, when viewed from outside, appears to be defect-free although there is no fusion between the metal sheets. It can be difficult to clearly detect a lack of fusion during the welding operation because that type of weld seam defect is predominantly inside the component or the weld seam and, as a result, indirect assessment variables may have to be used to detect certain defects.
In order to assess the quality of a weld seam during a laser welding operation, it is known to observe a solidified molten mass behind a liquid melting bath along the weld seam. For instance, U.S. Pat. No. 4,817,020 describes a process in which temperature is measured in real time at two or more mutually spaced apart locations of a solidified molten mass, and a cooling rate is established from the difference. Conclusions relating to the quality of the weld seam can be drawn from the cooling rate and interventions may optionally be made in the welding operation in order to optimize it.
It is further known from EP0655294 B1 to measure the temperature by means of high-speed pyrometers simultaneously and at both sides of a joint line at least at two locations behind a melting bath. Unlike temperature measurement at only one location, a clear association with process parameters of a welding operation can thereby be carried out. Point-like welding defects can further be detected at the measured locations.
In addition, the quality of a welding operation can also be assessed by monitoring a liquid melting bath of the welding operating. For instance, it is known, for example, from DE10338062A1, to monitor a liquid melting bath by means of a CCD camera during laser welding and to establish the relative, time-dependent movement of a front boundary face of the melting bath and a rear boundary face of a radiation surface of the laser. That measurement can be used to detect emissions from the melting bath.
It is further known from EP1119436A1 to establish the shapes of mutually spaced-apart maximum intensity regions and a minimum region of high-energy radiation, for example, plasma or laser radiation, which is located therebetween and to compare it with predetermined shapes in order thereby to control or adjust the material processing operation. The shapes of the spaced-apart maximum intensity regions and the minimum region of high-energy radiation can be established by technical measurements in a vapor capillary vessel.